Minimum Conductor Size for a Voltage-Drop Target
The inverse of the voltage-drop tile: the smallest standard copper and aluminum conductor that keeps the drop at or below a target percent (default 3%) over the run, from the load current, one-way length, and system voltage, with the resulting drop and an ampacity / termination reminder. The voltage-drop size is a floor, not a substitute for the 310.16 ampacity check.
Formula and source
allowed_drop_V = target% / 100 x source_V; for each standard size smallest-up, drop = (2 for single phase, sqrt(3) for three phase) x K x I x length / cmils with K copper 12.9, aluminum 21.2 ohm-cmil/ft at 75 C; the result is the first size whose drop is at or below the allowed value.
First-principles I x R voltage drop (public); NEC FPN advisory 3% branch / 5% total (informational, not a requirement).
Audience
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Posture
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